Yin Yang-1 (YY1) is a multifunctional transcription factor that derives it name from its ability to either repress or activate transcription depending upon DNA binding site context or cell type. YY1 is important for B cell development, as conditional knock-out of YY1 in the B cell lineage results in an early B cell defect with very few mature B cells. The initial block is at the pro-B cell stage and is associated with absence of immunoglobulin (Ig) locus large-scale chromosomal contraction resulting in a reduction in rearrangement of distal variable (V) region genes. However, other B cell defects are also evident because a rearranged Ig gene only partially reverts the mutant phenotype. Potential insight into the mechanistic basis for these effects is provided by our identification of YY1 as a mammalian Polycomb Group (PcG) protein. Interestingly, YY1 conditional knockout mice show a B cell defect phenotype similar to that seen by knock-out of the PcG protein, EZH2. We developed a YY1 mutant (YY1 REPO) that specifically ablates YY1 PcG function, and found that, in contrast to wild-type YY1 that efficiently rescued B cell development in a YY1-null bone marrow reconstitution system, the YY1 REPO mutant is incapable of supporting efficient B cell development. We also found that the YY1 REPO domain, necessary for YY1 PcG function, is able to physically interact with condensin and cohesin complex proteins involved in chromosomal contraction. Based upon these findings, and a variety of additional data, we hypothesize that YY1 recruits PcG and condensin proteins to the immunoglobulin loci resulting in the chromosomal contraction needed for rearrangement of distal variable region genes. Consistent with this hypothesis, we found that YY1 can bind to multiple sites within the Ig kappa locus and that, remarkably, EZH2 and condensin proteins colocalize with YY1 at these sites. Knock-down of condensin subunit SMC4 resulted in reduced Ig rearrangement suggesting that these interactions are directly related to Ig locus contraction. We will utilize the YY1 REPO mutant in conjunction with multiple novel YY1-null conditional knock-out systems to assess the role of YY1 PcG function in B cell development in vivo (Aim 1). We will also use our YY1-null reconstitution systems to determine the role of YY1 PcG function in Ig locus contraction and rearrangement of distal V region genes (Aim 2). Finally, we will explore the mechanism of YY1 function in immunoglobulin locus contraction (Aim 3). These studies will provide key mechanistic detail on the role of YY1 PcG function for B cell development and for processes critical for immune function. Understanding these functions of YY1 may provide multiple avenues for possible therapeutic intervention in diseases caused by PcG dysfunction, or aberrant stem cell self renewal.